1. Field of the Invention
The present invention relates to a method for fabricating organic electroluminescent (OEL) display devices, and more particularly to a low temperature polysilicon (LPTS) OEL display device and method for fabricating the same.
2. Description of Related Art
It is well known that display device is an important interface between a machine and its user for transmitting information. In recent years, flat panel display (FPD) devices have gradually replaced the known cathode radiation tube (CRT) display and become the main products of the display devices. The flat panel display (FPD) devices are popular in the market because of their advantages such as lightweight, small size and compactness. In addition, the rapid development of the information technology also induces a demand for a display device with high-resolution and high-volume-communication of information. However, the typical flat panel display (FPD) devices such as amorphous silicon thin-film-transistor liquid crystal display (xcex1-Si TFT LCD) are unable to meet the need for an efficient display. Under this circumstance, the display-industry began to develop a LTPS TFT (low temperature poly silicon thin film transistor) technique with superior performance to meet the demand of market. On the other hand, OEL devices also attract people""s attention for being a new generation of the FPD recently because of their advantages such as lightweight, high contrast, fast response time, low power consumption and high brightness. However, the technique of mass production of OEL has not yet become completely matured. Therefore, only-some samples made from the laboratory show up for demonstration. For commercialization and mass-production of the LTPS OEL devices, there are still a number of technical obstacles, which need to be solved.
Generally speaking, the conventional manufacturing method of LTPS TFTs includes thin film deposition, photolithography and etching techniques for fabricating the TFTs and pixel electrodes. Moreover, the LTPS TFTs require a special technique such as excimer laser annealing, to crystallize an xcex1-Si (amorphous silicon). After the excimer laser annealing process, doped atoms (dopants) are implanted into the LTPS TFTs at the appropriate sites such as a source, a drain, a lightly doped drain (LDD) and a channel thereof, as the conventional ion-implantation for processing polysilicon. Then, a high-temperature treatment by a furnace or rapid thermal annealing is proceeded to activate the dopants and crystallize the ion-damaged surface to rearrange to a polysilicon structure.
However, in the ion-implantation, some of the fast implanted ions will penetrate the polysilicon which has an ordered crystallographic orientation structure and be buried in another layer (e.g. substrate) next to the bottom of polysilicon. These penetrated implanted ions frequently result in device failure. So far, for achieving a good ion implantation by avoiding ions from penetrating through the exposed tunnels of the polysilicon and device failure caused by defects, the conventional ion-implantation is achieved by implanting ions in a tilted angle. This tilted angle technique include steps of controlling the angle of incidence of the ion beam with a tilt of five to ten degrees with respect to the normal and further reduces the possibility of exposing the tunnels. The penetration of implanted ions in the polysilicon layer can be also avoided by alternatively forming a film of different material over the polysilicon, such as oxide or xcex1-Si. The tunneling effect can be effectively reduced during ion implantation through scattering effect caused by the film of different material over the polysilicon. However, the above-mentioned processing of the methods are complicated and the efficiency of the process for mass-manufacturing LTPS OEL is greatly reduced.
Furthermore, the most appropriate process for mass production is a process with less thermal budget and less processing time. However, in the conventional production of the LTPS-TFT panel, a complementary metal oxide semiconductor (CMOS) process generally requires six to ten ion-implantation processing steps for different sites of a transistor element. A time-consuming and high-thermal-budget annealing for activating the dopants is also required after every ion-implantation for each site. Namely, for the conventional CMOS process, six to ten annealing steps for activating dopants are also required. This is a considerable time-consuming obstacle in the processes of mass production to be overcome as far as manufacturing efficiency and yield are concerned. Hence, it is a need to have a high efficient processing technique to meet the device needs and to increase the manufacturing yield.
It is therefore an object of the present invention to provide a method for fabricating LTPS OEL devices, which decreases the thermal budget for fabricating the LTPS OEL devices, reduces production cost, simplifies manufacturing processes and increases the efficiency of mass production of the OEL devices and yield.
It is a further object of the present invention to provide a method for fabricating LTPS OEL devices, which not only transforms xcex1-Si into polysilicon but also activates dopants by excimer laser annealing at the same time, wherein the dopants implanted into the xcex1-Si film in advance by ion-implantation serve as seeds so as to reduce the activation energy of crystallization and increase the efficiency of crystallization.
It is still a further object of the present invention to provide a method for fabricating LTPS OEL devices, which not only transforms xcex1-Si into polysilicon but also activates dopants by excimer laser annealing to prevent tunneling effect during the ion-implantation process at the same time after ions are implanted at every site of the grown xcex1-Si layer, wherein the dopants implanted into the xcex1-Si film in advance by ion-implantation serve as seeds so as to reduce the activation energy of crystallization and increase the efficiency of crystallization.
A method for fabricating a LTPS OEL substrate according to the present invention comprises the following steps: providing a substrate; forming an amorphous silicon layer on said substrate; forming a plurality of patterned transistor element each having a patterned source, a patterned drain and a patterned gate on said amorphous silicon layer through photolithography and ion doping; annealing said transistor elements having said patterned sources, drains and gates by direct radiation of excimer laser; forming a plurality of patterned stripes of second conductive lines connected to said gates on the surface of said substrate; forming a patterned isolation layer on said gate layer and part of said second conductive lines, and also forming a plurality of patterned stripes of first conductive lines and a patterned first electrode on said substrate wherein said first and said second conductive lines sandwich said isolation layer, said first conductive lines are connected to said sources and said first electrode is connected to said drains; forming at least one organic electroluminescent medium on said first electrode; and forming a second electrode layer on said organic electroluminescent medium; wherein said first conductive lines and said second conductive lines are intersected without directly electrical contact.
To illustrate the present invention, exemplary embodiments of a method for fabricating LTPS OEL devices will now be described with reference to the accompanying drawings.